It is the goal of this research plan to elucidate the renal mechanism responsible for the concentrating defect due to hypokalemia and hypercalcemia. Experiments will be performed using conventional micropuncture techniques using Munich-Wistar rats. The parameters to be measured will include: papillary plasma flow; measurement of urea recycling in superficial and juxtamedullary nephrons, measurement of osmotic equilibration secondary to antidiuretic hormone in cortical and juxtamedullary nephrons; to determine whether an acute urea infusion can correct the concentrating defect of hypokalemia and hypercalcemia and finally, an in vivo study of unidirectional permeability measurements to water and urea and net osmotic water flow of papillary collecting ducts in the presence and absence of antidiuretic hormone. Unidirectional permeability coefficients will be measured using conventional tracer techniques. Osmotic water flow will be examined by perfusing the lumen with osmolalities of approximately 200 mosm less than the papillary interstitium and then calculating the Lp by routine equations.